Maxwell's Demon Claims There's A Way Around This Law Of Physics

Brilliance | Dec. 17, 2017

Physics is full of strange laws and unintuitive rules that can be tricky to test in a laboratory. That's why it's also full of thought experiments: fun "what if" scenarios that illuminate an otherwise murky element of the science (Schrödinger's Cat, anyone?). For instance, how much do you know about the second law of thermodynamics? Think of it in terms of Maxwell's Demon, and you'll understand a bit better.

You Gotta Pay The Troll Toll

This thought experiment was first introduced by the Scottish physicist James Clerk Maxwell in his 1871 book "Theory of Heat." In what's been called one of the most heavily quoted passages in physics, Maxwell begins by laying out the second law of thermodynamics. The relevant portion? If you have an even distribution of temperature and pressure in a closed system, you can't force that distribution to become uneven without doing work — that is, exerting energy.

Maxwell wants you to imagine that there's a really smart little being (his "demon," though that name came after the thought experiment). The demon can watch individual molecules to tell which ones are hot (fast moving) and which ones are cold (slow moving). He's not saying, "Imagine magic is real!"; his so-called demon is just a metaphor for the kind of advanced technology he believed we might one day have for this kind of thing.

Maxwell uses a gas-filled chamber in this thought experiment, but for a real-world example, pretend there's a swimming pool. It's separated in the middle by a solid wall. The water is the same temperature on both sides, which means that it has about the same mix of fast-moving and slow-moving molecules, but water can't pass through the wall except through one single molecule-sized opening.

The demon — wearing swim trunks and a snorkel, no doubt — sits by the opening and spots all of the fast- and slow-moving molecules. Every time he catches a hot, fast molecule, he sends it into side B. Every time he catches a cold, slow molecule, he sends it into side A. "He will thus, without expenditure of work," Maxwell concludes, "raise the temperature of B and lower that of A, in contradiction to the second law of thermodynamics."

Maxwell used this thought experiment to point out that some "laws" of the universe might only exist because of our scientific limitations. Scientists of the day didn't have the technology to pinpoint and control individual molecules, but if they did, Maxwell says, they could contradict the second law of thermodynamics. That would completely change our approach to energy. If you could harness the power of heat without using any energy in the first place, that would mean free energy. We wouldn't need oil or solar panels or nuclear power; technology as primitive as steam engines could run forever without requiring fuel. We've had more than a century of technological progress since Maxwell's thought experiment, so where are our perpetual motion machines?

The Demon Is In The Details

As you might have guessed, there are some problems with Maxwell's Demon. The biggest objection is this one: if the "demon" were a real device, and not some magical being, then it would use energy to detect the individual molecules and to trigger the opening between the chambers. Specifically, the demon would actually use more energy to identify and move the molecules than you could get out of the final product.

Of course, this thought experiment wouldn't keep popping up in scientific journals if it were that easily debunked. Scientists continue to debate and test the nitty-gritty details of Maxwell's Demon and there has been some progress, especially in the realm of quantum physics. Maybe one day, we'll be as smart as Maxwell's little creature. You'd never have to charge your phone again!